Radiating structural body of electronic part and radiating sheet used for the radiating structural body

ABSTRACT

A heat dissipating structure for a heat generating electronic component is characterized by comprising a heat dissipating sheet including a metal sheet and a heat conductive member having adhesion stacked thereon between the heat generating electronic component and a heat dissipating member, wherein the metal sheet is connected to the heat generating electronic component, and the heat conductive member having adhesion is connected to the heat dissipating member.

TECHNICAL FIELD

[0001] This invention relates to a heat dissipating structure which caneffectively conduct the heat generated by heat generating electroniccomponents such as transistors and computer CPUs to heat dissipatingmembers such as heat sinks, and a heat dissipating sheet used therein.

BACKGROUND ART

[0002] The recent drive for higher integration and operating speeds inCPUs, driver ICs, memories and other LSIs used in electronic equipmentsuch as personal computers, digital video disks and mobile phones hasincreased the power consumption. Concomitantly the heat release hasincreased, which can cause malfunction of electronic equipment ordamages to electronic components. It becomes very important to addressthe heat dissipation.

[0003] In prior art electronic equipment, heat sinks in the form ofplates made of brass and other high thermal conductivity metals are usedin order to suppress a temperature rise of electronic components duringoperation. The heat sinks conduct the heat generated by the electroniccomponents and release the heat from their surface by means of atemperature difference from the ambient air.

[0004] For effective heat transfer from electronic components to heatsinks, the heat sink must be placed in intimate contact with theelectronic component. Because of height differences among variouselectronic components and crossings in the assembly process, a flexibleheat conductive sheet or a heat conductive grease is interposed betweenthe electronic component and the heat sink so that the heat transferfrom the electronic component to the heat sink is established throughthe heat conductive sheet or heat conductive grease. The heat conductivesheet used is typically a heat conductive sheet made of heat conductivesilicone rubber or the like (i.e., heat conductive silicone rubbersheet), and the heat conductive grease used is typically a heatconductive silicone grease.

[0005] The conventionally used heat conductive silicone rubber sheet,however, has a substantial contact thermal resistance at the interfacewith an electronic component, which imposes a certain limit to the heattransfer performance. This is a serious problem when electroniccomponents of high-frequency drive with substantial heat generation suchas CPUs are to be cooled. It is desired to reduce the interfacialcontact thermal resistance.

[0006] On the other hand, heat conductive grease has a substantiallynegligible level of interfacial contact thermal resistance because ofits liquid-like nature and exerts good heat transfer performance, butsuffers from the problems that a special equipment such as dispenser isneeded and its recovery entails an inefficient operation.

[0007] To overcome some of the above-described problems, a phase changeheat dissipating sheet (simply phase change sheet) was proposed which isa solid sheet at normal room temperature, but softens by the heatgenerated by a electronic component during operation, to reduce theinterfacial contact thermal resistance to a negligible level. Thefollowing phase change sheets were proposed in the prior art. U.S. Pat.No. 4,466,483 discloses a non-metallic sheet having phase change waxlayers formed on opposite surfaces thereof. Also, U.S. Pat. No.5,904,796 discloses a metal foil having a phase change paraffin orpetroleum jelly formed on one surface thereof and an adhesive formed onthe other surface thereof. JP-A 2000-509209 discloses a phase changesheet characterized by comprising an acrylic pressure-sensitiveadhesive, a wax and a heat conductive filler, with an intermediate layerof network structure, film or the like being excluded.

[0008] On the other hand, the heat dissipating sheet is, on use,interposed between a heat dissipating member such as a heat sink and aheat generating electronic component such as CPU. In the recentindustry, a situation has arisen where the process of producing personalcomputer components and the process of assembling such components tocomplete personal computers are separately performed at differentlocations over the world. It thus becomes a routine to attach a heatdissipating sheet to a heat sink at one location and transport theassembly to another location where it is mounted to CPU.

[0009] When the heat dissipating sheet-attached heat sink assembly is tobe inspected for quality, the assembly must be mounted to a model CPUand subjected to a burn-in test or thermal cycling test under the samestate as actual mounting. In the test, the phase change sheet works wellbecause it melts at a temperature above the melting point and comes inclose contact with the heat sink and the CPU. However, the phase changesheet can fail when the heat sink is separated from the CPU after thetest. Even the electronic component and the heat sink can also failbecause of strong adhesive bonds. In the event of failure of the phasechange sheet, not only a new phase change sheet must be attached again,but a test must be done again to inspect whether the new phase changesheet meets the desired quality, which is unreasonable. After assemblyto a personal computer, a burn-in test or thermal cycling test issometimes done, giving rise to the same problem.

[0010] Moreover, U.S. Pat. No. 5,550,326 disclose a heat dissipator orheat dissipating structure in which an adhesive heat conductive padlayer is formed on one surface of a metal foil and which is used suchthat the surface of the adhesive heat conductive pad layer is in closecontact with the electronic component side and the surface of the metalfoil is in close contact with a heat dissipating member. This structurehas the problem that since the adhesive heat conductive pad is in closecontact with the electronic component, an attempt to peel the adhesiveheat conductive pad from the electronic component can result in failureof the adhesive heat conductive pad and even failure of the electroniccomponent.

[0011] The present invention intends to improve the above-discussedsituation and its object is to provide a heat dissipating structure fora heat generating electronic component, which can effectively conductthe heat generated by the electronic component to a heat dissipatingmember such as a heat sink, and which even after a burn-in test orthermal cycling test to be performed for quality inspection, allows theelectronic component and the heat dissipating member to be separatedwithout structural failure of a heat dissipating sheet or failure of theelectronic component, as well as a heat dissipating sheet used therein.

DISCLOSURE OF THE INVENTION

[0012] To attain the above object, the present invention provides a heatdissipating structure for a heat generating electronic component,characterized by comprising a heat dissipating sheet including a metalsheet and a heat conductive member having adhesion stacked thereonbetween the heat generating electronic component and a heat dissipatingmember, wherein the metal sheet is connected to the heat generatingelectronic component, and the heat conductive member having adhesion isconnected to the heat dissipating member. The present invention alsoprovides a heat dissipating sheet interposed between a heat generatingelectronic component and a heat dissipating member, characterized bycomprising a metal sheet to be disposed on the side of the heatgenerating electronic component and a heat conductive member havingadhesion stacked on the metal sheet and to be disposed on the side ofthe heat dissipating member, wherein the heat conductive member havingadhesion is formed of a heat conductive composition comprising at leastone resin component selected from among siloxane polymers, acrylicpolymers and polyolefin polymers, and a heat conductive filler, thecomposition being able to phase change or soften by the heat from theelectronic component during operation.

[0013] According to the invention, heat can be effectively conductedfrom the heat generating electronic component to the heat dissipatingmember, typically a heat sink. Even after a burn-in test or thermalcycling test, the electronic component or heat dissipating member can beremoved from the heat dissipating sheet without inconvenient problemsincluding structural failure of the heat dissipating sheet and failureof the electronic component.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-sectional view of a heat dissipating sheetaccording to one embodiment of the invention.

[0015]FIG. 2 is a cross-sectional view of a heat dissipating structureaccording to one embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] Now the present invention is described in further detail.

[0017] The heat dissipating sheet of the invention is to be interposedbetween a heat generating electronic component and a heat dissipatingmember and is characterized by comprising a metal sheet to be disposedon the side of the heat generating electronic component and a heatconductive member having adhesion stacked on the metal sheet and to bedisposed on the side of the heat dissipating member, wherein the heatconductive member having adhesion is formed of a heat conductivecomposition comprising at least one resin component selected from amongsiloxane polymers, acrylic polymers and polyolefin polymers, and a heatconductive filler, the composition being able to phase change or softenby the heat from the electronic component during operation.

[0018] The preferred metal sheet used herein is an aluminum sheet,copper sheet, stainless steel sheet, tungsten sheet, gold sheet or thelike, with the aluminum and copper sheets being more preferred for lowcost. The metal sheet preferably has a thickness in the range of 3 to200 μm, more preferably in the range of 5 to 75 μm. With a thickness ofless than 3 μm, strength may be short. A thickness in excess of 200 μmmay adversely affect the flexibility of the heat dissipating sheet,detracting from close contact with the electronic component.

[0019] The heat conductive member used herein may be formed of aclay-like heat conductive composition having pressure-sensitiveadhesion, typical of which is an acrylic base pressure-sensitiveadhesive, siloxane base pressure-sensitive adhesive or olefin basepressure-sensitive adhesive in which a heat conductive filler iscompounded.

[0020] Preferably the clay-like heat conductive composition has aplasticity at 25° C. in the range of 100 to 1,000. With a plasticity ofless than 100, the heat conductive member may have low strength andbecome less convenient to handle. With a plasticity of more than 1,000,the heat dissipating sheet may lack flexibility, giving rise to aproblem to the close contact with the heat dissipating member.

[0021] In a preferred embodiment, the adhesive heat conductive membercontains a phase transition material or thermosoftening material,because in response to heat generation of the electronic componentduring operation, the heat conductive member changes its phase from thesolid to a liquid or fluid/semifluid or thermally softens for reducingthe contact thermal resistance between it and the heat dissipatingmember. The phase transition or thermosoftening materials used hereininclude paraffin waxes, α-olefins, silicone resins, and fluoro-resins.These phase transition or thermosoftening materials preferably have anendothermic peak associated with phase change or thermal softening asmeasured by a differential scanning calorimeter (DSC) in the range of 35to 12° C., more preferably 40 to 100° C., especially the operatingtemperature range of electronic components of 50 to 80° C.

[0022] The heat conductive fillers used in the heat conductive memberinclude metal powders such as iron, aluminum, nickel, silver, and gold,inorganic oxide powders such as silicon oxide, aluminum oxide, zincoxide, iron oxide, and magnesium oxide, and inorganic nitride powderssuch as aluminum nitride and boron nitride. The heat conductive fillermay have an average particle size in the range of 0.1 to 30 μm, with themaximum particle size being preferably up to 100 μm.

[0023] It is understood that the amount of the phase transition orthermosoftening material compounded and the amount of the heatconductive filler compounded in the acrylic, siloxane or olefin basepressure-sensitive adhesive may be determined as appropriate. Preferablythe amount of phase transition or thermosoftening material is 10 to1,000 parts by weight, especially 50 to 500 parts by weight per 100parts by weight of the adhesive, and the amount of heat conductivefiller is 0 to 3,000 parts by weight, especially 100 to 2,000 parts byweight per 100 parts by weight of the adhesive.

[0024] With respect to the thickness of the adhesive heat conductivemember, a thinner member is advantageous because the thermal resistancebecomes lower. However, it is substantially impossible to reduce thethickness below the maximum particle size of the heat conductive filler.Thus the thickness preferably ranges from a value equal to the maximumparticle size of the heat conductive filler to an approximate 5 fold,and typically from 0.1 to 1,000 μm, especially from 1 to 500 μm.

[0025] The method of preparing the heat dissipating sheet comprising ametal sheet and a heat conductive member formed on one surface thereofis, for example, by dissolving the heat conductive composition in anorganic solvent and applying the resulting slurry by any well-knowncoating or spraying technique. FIG. 1 illustrates the construction ofthe heat dissipating sheet of the invention. In FIG. 1, 1 denotes theheat dissipating sheet, 1-a denotes a heat conductive member, and 1-bdenotes a metal sheet.

[0026] In another aspect, the present invention provides a heatdissipating structure for a heat generating electronic component,comprising the above-described heat dissipating sheet including a metalsheet and an adhesive heat conductive member stacked thereon between theheat generating electronic component and a heat dissipating member, themetal sheet being connected to the heat generating electronic component,and the adhesive heat conductive member being connected to the heatdissipating member. As shown in FIG. 2, this structure is constructed ashaving a heat dissipating sheet 1 interposed between an electroniccomponent 2 and a heat dissipating member 3. The heat conductive member1-a is connected to the heat dissipating member 3, and the metal sheet 1b connected to the electronic component 2. With these connections, evenafter a burn-in test or thermal cycling test to be performed for qualityinspection, the electronic component and the heat dissipating member canbe removed without structural failure of the heat dissipating sheet.Since the heat conductive member in this heat dissipating structure hasan adequate degree of pressure-sensitive adhesion, the heat dissipatingsheet can be removed as being attached to the heat dissipating member.Moreover, since the adhesive heat conductive member is reinforced withthe metal sheet, the heat dissipating sheet can be removed from the heatdissipating member without structural failure of the sheet, ensuringeasy repair.

EXAMPLE

[0027] Examples are given below for illustrating the invention, but theinvention is not limited to the examples.

Examples 1-6

[0028] 20 parts by weight of xylene was added to 100 parts by weight ofa heat conductive composition of the formulation shown in Table 1 toform a slurry, which was applied onto a metal sheet by means of a barcoater, and dried at 80° C. for 20 minutes, obtaining a heat dissipatingsheet of 0.1 mm thick.

[0029] The heat dissipating sheet was measured for physical propertiesby the following measurement methods, with the results shown in Table 1.

[0030] It is seen from these results that the heat dissipating structureand heat dissipating sheet of the invention are very useful.

[0031] Measurement Methods

[0032] 1) Plasticity Measurement:

[0033] Measured by the plasticity test of JIS K-6249

[0034] 2) Thermal Conductivity Measurement:

[0035] Measured by a thermal conductivity meter QTM-500 (trade name,Kyoto Denki Co., Ltd.)

[0036] 3) Thermal Resistance Measurement:

[0037] A sample of 0.5 mm thick punched into a TO-3 transistor shape wasinterposed between a transistor 2SD923 (trade name, Fuji Electric Co.,Ltd.) and a heat sink FBA-150-PS (trade name, OS Co., Ltd.), and acompression load of 1000 gf/cm² was applied. The heat sink was placed ina thermostat water tank and held at 60° C.

[0038] Then a power of 10 V and 3 A was fed to the transistor. After 5minutes, the temperature (T1) of the transistor and the temperature (T2)of the heat sink were measured using thermocouples embedded in thetransistor and heat sink. The thermal resistance Rs (in ° C./W) of thesample was computed according to the equation: Rs=(T1−T2)/30.

[0039] 4) Adhesive Strength Measurement:

[0040] Measured by the adhesive tape test of JIS Z-0237

[0041] 5) Thermosoftening Temperature:

[0042] Measured from an endothermic peak by DSC

[0043] 6) Repair:

[0044] At the end of the above thermal resistance measurement, thetransistor and the heat sink were separated apart. The heat dissipatingsheet was rated OK (repairable) when it was kept connected to the heatsink side with its shape retained. TABLE 1 Compounding amount (pbw)Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Acrylicadhesive 100 0 0 50 0 0 Silicone adhesive 0 100 0 0 50 0 Olefin adhesive0 0 100 0 0 50 α-olefin 0 0 0 0 0 50 Paraffin wax 0 0 0 50 0 0 Siliconeresin 0 0 0 0 50 0 Heat conductive filler 1 1400 1400 1400 0 0 0 Heatconductive filler 2 0 0 0 800 800 800 Metal sheet Al foil Al foil Alfoil Cu foil Cu foil Cu foil 0.05 mm 0.05 mm 0.05 mm 0.018 mm 0.018 mm0.018 mm Thickness, mm 0.1 0.1 0.1 0.1 0.1 0.1 Thermal conductivity,W/mK 3.2 3.1 3.0 4.1 4.0 3.9 Thermal resistance, ° C./W 0.06 0.06 0.070.03 0.03 0.03 Thermosoftening temp., ° C. — — — 55 59 65 Adhesivestrength, N/cm 0.60 0.45 0.17 0.13 0.15 0.10 Plasticity 300 310 400 500600 700 Repair OK OK OK OK OK OK

[0045] Materials Used in Examples

[0046] Acrylic pressure-sensitive adhesive: SK-DYNE 1345 (trade name,Soken Chemical & Engineering Co., Ltd.)

[0047] Silicone pressure-sensitive adhesive: KR130 (trade name,Shin-Etsu Chemical Co., Ltd.)

[0048] Olefin pressure-sensitive adhesive: Lucant HC3000X (trade name,Mitsui Chemical Co., Ltd.)

[0049] α-olefin: DIALEN 30 (trade name, Mitsubishi Chemical Corp.)

[0050] Paraffin wax: 130 (trade name, Nippon Seiro Co., Ltd.)

[0051] Silicone resin: X-40-9800-59 (trade name, Shin-Etsu Chemical Co.,Ltd.)

[0052] Heat conductive filler 1: alumina AO-41R (trade name, AdmatechsCo., Ltd.)

[0053] Heat conductive filler 2: silver powder Ag-E-100 (trade name,Fukuda Metal Foil/Powder Industry Co., Ltd.)

[0054] Aluminum foil, copper foil: made by Fukuda Metal Foil/PowderIndustry Co., Ltd.

[0055] The heat dissipating sheet for a heat generating electroniccomponent according to the present invention can effectively conduct theheat generated by the electronic component to a heat dissipating membersuch as a heat sink, and even after a burn-in test or thermal cyclingtest to be performed for quality inspection, allows the electroniccomponent and the heat dissipating member to be separated withoutstructural failure of the heat dissipating sheet or failure of theelectronic component.

1. A heat dissipating structure for a heat generating electroniccomponent, characterized by comprising a heat dissipating sheetincluding a metal sheet and a heat conductive member having adhesionstacked thereon between the heat generating electronic component and aheat dissipating member, wherein the metal sheet is connected to theheat generating electronic component, and the heat conductive memberhaving adhesion is connected to the heat dissipating member.
 2. The heatdissipating structure of claim 1 wherein the metal sheet is an aluminumsheet or copper sheet.
 3. The heat dissipating structure of claim 1 or 2wherein the heat conductive member having adhesion is clay-like.
 4. Theheat dissipating structure of any one of claims 1 to 3 wherein the heatconductive member having adhesion contains a phase transition materialor thermosoftening material.
 5. A heat dissipating sheet interposedbetween a heat generating electronic component and a heat dissipatingmember, characterized by comprising a metal sheet disposed on the sideof the heat generating electronic component and a heat conductive memberhaving adhesion stacked on the metal sheet and disposed on the side ofthe heat dissipating member, wherein the heat conductive member havingadhesion is formed of a heat conductive composition comprising at leastone resin component selected from among siloxane polymers, acrylicpolymers and polyolefin polymers, and a heat conductive filler, thecomposition being able to phase change or soften by the heat from theelectronic component during operation.